Abstract (Description) Liver diseases, including nonalcoholic steatohepatitis (NASH) and alcoholic liver disease (ALD), are devastating and reduce lifespan and quality of life. Hepatocyte and cholangiocyte (bile duct epithelial cells) injury and death are hallmarks of all liver diseases. In NASH and ALD, excessive lipid accumulation (steatosis) induces hepatocyte injury. Damaged hepatocytes and cholangiocytes activate Kupffer cells and macrophages (? cells), leading to liver inflammation. Damaged hepatocytes and cholangiocytes also stimulate, in concert with activated ? cells, differentiation of hepatic stellate cells and portal fibroblasts into active myofibroblasts that are responsible for liver fibrosis. In order to prevent/treat NASH and ALD, it is imperative to understand the liver injury-sensing machinery that orchestrates the destructive programs responsible for liver damage, inflammation, and fibrosis. Excitingly, we found that NF-?B-inducing kinase (NIK, also called MAP3K14) is a critical component of the machinery. NIK is known to activate the noncanonical NF-?B2 pathway and promote immune organ development. Abnormally-activated NIK promotes cancer development. However, NIK function in the liver is poorly defined. We found that hepatic NIK is highly activated in mice and humans with NASH, ALD, and drug-induced liver injury. Similarly, biliary NIK is also highly activated in biliary injury. To assess the role of liver NIK, we deleted NIK specifically in hepatocytes and cholangiocytes using inducible loxp/Cre systems. We showed that liver-specific deletion of NIK protects against high fat diet-induced liver steatosis and hepatotoxin-induced liver injury and inflammation; conversely, hepatocyte-specific overexpression of NIK induces liver injury, inflammation, and fibrosis. Furthermore, cholangiocyte-specific deletion of NIK attenuates toxin-induced bile duct proliferation and biliary fibrosis. Remarkably, treatment with small molecule NIK inhibitors substantially attenuates hepatotoxin-induced liver injury. At the molecular level, we found that NIK suppresses the STAT3 pathway that promotes hepatocyte survival. A subset of NIK localizes to mitochondria and binds to Drp1, a master regulator of mitochondrial fission. In light of these exciting findings, we hypothesize that liver NIK is a central component of the liver injury-sensing machinery. It shapes hepatocyte and cholangiocyte behavior, at least in part, through regulating the STAT3 and Drp1/mitochondria pathways. Furthermore, intrinsic NIK induces hepatocytes and cholangiocytes to release factors that stimulate ? cells, and activated ? cells further activate hepatic and biliary NIK through positive feedback loops, thereby driving liver disease progression. We will test these hypotheses in three Aims. Aim 1: Assess the role of hepatic NIK in NASH and ALD. Aim 2: Define the role of biliary NIK in bile duct proliferation and biliary fibrosis. Aim 3: Delineate the underlying molecular mechanisms responsible for the pathogenic actions of hepatic and biliary NIK. The outcomes are expected to define a critical pathogenic role of liver NIK and provide proof of concept evidence for the therapeutic potential of NIK inhibitors for NASH and ALD.